Chondrocyte

Chondrocytes are the only cells in hyaline cartilage, occupying a low single-digit percentage of tissue volume and sitting singly or in small clusters inside lacunae within the matrix they secrete. They live in a genuinely hypoxic niche, since the tissue has no vessels and oxygen must diffuse from synovial fluid, and they rely on glycolysis and hypoxia-inducible factor signalling. The transcription factor SOX9 maintains their identity and drives expression of type II collagen and aggrecan. In adult cartilage they divide essentially not at all.

In osteoarthritis chondrocytes do not simply die quietly; they change phenotype. They enlarge and begin expressing markers of the growth-plate hypertrophic programme, including type X collagen, alongside the degradative enzymes MMP-13 and ADAMTS-5, which cleave type II collagen and aggrecan respectively. The cell that maintains the matrix becomes the agent of its destruction, which is why aggrecanase and MMP-13 inhibition were pursued as disease-modifying targets.

Culture behaviour is the reason to distrust in-vitro chondrogenic claims. Chondrocytes plated in monolayer dedifferentiate within a few passages, losing SOX9 and type II collagen expression and switching to type I collagen, becoming effectively fibroblast-like.

So chondroprotection inferred from a dish deserves little weight. A peptide that increases proliferation or proteoglycan staining in cultured chondrocytes has usually been tested on partly dedifferentiated cells, at concentrations chosen for the assay rather than matched to anything achievable in synovial fluid, with no evidence it reaches cartilage in an intact joint. Proliferation is also the wrong merit criterion: adult cartilage repair fails because the matrix is not rebuilt, not because too few cells divided.